Step 1: Recall what a tsunami is.
A tsunami is a set of very long ocean waves made when a large volume of water is suddenly displaced, usually by an undersea earthquake.
Step 2: Look at deep water behaviour.
In the deep open ocean the wave spreads its energy through a great depth, so its wavelength becomes extremely long, often hundreds of kilometres.
Step 3: Look at its height there.
Over deep water the same wave has only a small height, often under a metre, so ships barely feel it pass.
Step 4: Note what changes near shore.
Only when the wave reaches shallow coastal water does it slow, pile up and grow tall.
Step 5: Match the deep water case.
So in deep water the wave has a very long wavelength and limited (small) height.
Step 6: Conclude.
Over deep ocean water a tsunami has a very long wavelength and limited wave height.
\[ \boxed{\text{Very long wavelength and limited wave height}} \]